Cut out stem ends of bell peppers and discard. Scoop out seeds. Bring 8 cups water to a boil in a large pot and blanch the peppers until tender-crisp, about 1 minute. Drain and cool under cold running water. Set aside.

Heat oil in a large nonstick skillet over medium heat. Add onion and garlic and cook, stirring occasionally, until softened, about 3 minutes. Add turkey and cook, crumbling with a wooden spoon, just until it loses its pink color, about 2 minutes. Drain the fat.

Transfer the turkey mixture to a medium bowl and mix in rice, 1/2 cup tomato sauce, parsley, salt (if using) and pepper. Stuff the peppers with the mixture and place them in a 2-quart casserole dish. Spoon the remaining 1/2 cup tomato sauce over the peppers. Cover and bake until the peppers are tender and the filling is heated through, 30 to 35 minutes.